Organic and inorganic peroxygen compounds, such as peroxides, hydroperoxides, peresters, persulfates, hydrogen peroxide, etc. are widely employed as free-radical initiators in polymerizations of ethylenically unsaturated compounds. However, peroxygen initiators have shortcomings which limit their usefulness. They often oxidize polymers or leave oxygenated residues which cause discoloration on aging and exposure to light or they alter the color of dyestuffs. Many are shock sensitive and unpredictable in behavior and can cause undesirable chain-branching.
On the other hand, azo initiators such as the azonitriles disclosed in U.S. Pat. No. 2,471,959 and compounds derived from them do not have the undesirable characteristics ascribed to peroxides and are highly useful in vinyl polymerizations. Azo initiators which are water soluble are particularly useful in aqueous solution and emulsion polymerization. However, most water soluble azo compounds have salt-like functional groups (see, for example, U.S. Pat. No. 2,520,338 and U.S. Pat. No. 2,599,300) which introduce functional groups into polymers which can reduce thermal stability as well as detract from physical electrical and mechanical properties. Water insoluble azo compounds such as azodinitriles are known to possess free-radical initiation properties in some aqueous emulsion polymerization systems but are difficult to introduce into aqueous polymerization systems where organic solvents cannot be tolerated. Such azodinitriles decompose to form toxic byproducts. Water soluble azo compounds such as azodiamides are known to possess free-radical initiation properties at high temperatures where most polymerizations are carried out. However, at lower temperatures where the best polymers are made, the azodiamides are not effective. Additionally, the azodiamides decompose to form toxic byproducts. A initiator that is water soluble, effective at lower polymerization temperatures and which possess more favorable toxicity properties is desirable.